The invention relates to a data carrier which can be operated in a contact-bound mode and in a contactless mode and which includes the means specified hereinafter, namely transmission contacts which are mechanically accessible from outside the data carrier and of which a first transmission contact serves to receive a clock signal in the contact-bound mode of the data carrier, and at least one transmission coil which is inductively accessible from outside the data carrier and which serves to receive a useful HF signal in the contactless mode of the data carrier, and a circuit which includes a.c. detection means constructed to detect the presence of a received useful HF signal, by which a.c. detection means the supply of contactless activation information can be initiated upon the detection of the presence of such a useful HF signal in a detection process, which information enables the contactless mode to be activated.
The invention further relates to a circuit for a data carrier which can be operated in a contact-bound mode and in a contactless mode and which includes the means specified hereinafter, namely contact terminals which are adapted to be connected to transmission contacts which are mechanically accessible from outside a data carrier and of which a first contact terminal serves to receive a clock signal in the contact-bound mode of the circuit, and two coil terminals which serve for connection to a transmission coil which is inductively accessible from outside a data carrier and for receiving a useful HF signal in the contactless mode of the circuit, and a.c. detection means constructed to detect the presence of a received useful HF signal, by which a.c. detection means the supply of contactless activation information can be initiated upon the detection of the presence of such a useful HF signal a detection process, which information enables the contactless mode to be activated.
Such a data carrier of the type defined in the first paragraph and such a circuit of the type defined in the second paragraph are known, for example from the document WO 96/38814 A1.
The known data carrier includes only the a.c. detection means, which in this case are realized by an a.c. detection circuit which is connected directly to a transmission coil and which activates the contactless mode and thus a contactless interface when a useful HF signal appears across the transmission coil. A problem encountered with the known data carrier is that in an operating condition in which the data carrier is in the contact-bound mode and powerful spurious signals are received via the transmission coil the received spurious signals fake a received useful HF signal, which is detected as the appearance of a useful HF signal by the a.c. detection circuit, as a result of which in response to the received spurious signal the contactless mode is activated erroneously, although the data carrier has been set to and should remain in the contact-bound mode. Thus, in the case of the known data carrier the operation in the contact-bound mode is interrupted, in response to a spurious signal received by means of the transmission coil, which in the case that a data transmission process is in progress in the contact-bound mode results in this process being disrupted. Such an interruption or disruption of an activated mode of operation as a result of an undesired activation of the other mode of operation is obviously undesirable and inadmissible.